Processes for manufacture of a dry strength paper and methods for imparting dry strength to paper using a cationic starch graft polymer

ABSTRACT

Embodiments of the present disclosure provide for processes for manufacture of a dry strength paper, methods for imparting dry strength to paper using a cationic starch graft polymer, and the like.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of Ser. No. 12/699,116; filed Feb. 3,2010; which is a Divisional of U.S. application Ser. No. 10/788,064filed Feb. 26, 2004; issued as U.S. Pat. No. 7,786,238 on Aug. 31, 2010;which claims priority to German Application 10308753.2; all of which arehereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to cationic starch graft copolymers whichare suitable for increasing the dry strength of paper, their use andtheir preparation.

The dry strength of paper covers various strength parameters which aredetermined on the dry paper, such as dry tensile strength, burstingstrength, cleavage strength or strength in the z direction, stiffness,buckling resistance and surface strength (cf. “Dry Strength Additives”,Ed. W. F. Reynolds, TAPPI Press 1980, Atlanta). For increasing the drystrength, certain additives can be added to paper. The dry strength isdetermined primarily by hydrogen bridges within the paper sheet, inaddition to the natural strength of the cellulose fibres themselves.Polymeric, hydrophilic additives which can form hydrogen bridges withthe cellulose fibres and thus increase the contact area between thecellulose fibres have therefore proved useful. Typical examples of theseare starch, galactomannans, polyacrylamides, carboxymethylcellulose andpolyvinyl alcohol.

Additives for increasing the dry strength, i.e. dry strength agents, canbe added to the paper pulp and/or applied to the surface of the paper.For use in the pulp, either cationic, self-retaining polymers areemployed or anionic or amphoteric polymers are used in combination witha cationic fixing agent. In the case of surface use, the charge plays norole for the fixing, so that virtually uncharged polymers, such aspolyvinyl alcohol, can also be used here. Starches and galactomannanswhich are used as dry strength agents in the pulp are usuallycationically modified. Polyacrylamides are used in anionic, amphotericor cationic form, amphoteric or anionic polyacrylamides usually beingcombined with a fixing agent when used in the pulp in order to achievegood retention in the paper.

U.S. Pat. No. 4,400,496 describes graft polymers based on starch andacrylamide, starch serving as the grafting base and the grafts eitherconsisting only of acrylamide, i.e. being nonionic, or being composed ofacrylamide and acrylic acid, i.e. being anionic. If they are added asdry strength agents to the paper pulp, such products must be combinedwith a fixing agent in order to ensure sufficient retention. EP-A 194987 describes cationic starch graft polymers as paper dry strengthagents, starch being used as grafting base and the grafts consisting ofcationic polyacrylamide fragments. This is achieved by grafting onacrylamide together with a cationic vinyl monomer. The productsdescribed can be used without a fixing agent in the paper pulp sincethey have sufficient intrinsic retention owing to the cationic charge. Asignificant increase in the dry breaking strength is achieved thereby.

DESCRIPTION OF THE INVENTION

It has now surprisingly been found that cationic starch graft polymerswhose grafts preferably predominantly comprise acrylamide and/ormethacrylamide can be further improved in their action as dry strengthagents if crosslinking agents having a functionality of 2 or higher areconcomitantly used during the graft polylmerization of acrylamide ormethacrylamide, cationic vinyl monomer and optionally furthermonofunctional vinyl monomers.

The present invention therefore relates to cationic starch graftpolymers comprising the monomers a) to d)

-   a) 20-80% by weight of acrylamide, methacrylamide or mixtures    thereof,-   b) 3-20% by weight of at least one basic or cationic vinyl monomer,-   c) 0.005-1.5% by weight of at least one bifunctional or    higher-functional crosslinking agent,-   d) 0-10% by weight of at least one nonionic or anionic vinyl monomer    which differs from the monomers mentioned under a) to c)    and a grafting base e)-   e) 15-70% by weight of at least one starch or one starch derivative,    the sum of a) to e) being 100% by weight.

The starch graft polymers according to the invention are obtainable byfree radical polymerization of monomers a) to d) in the presence of thegrafting base e).

Acrylamide, methacrylamide and any desired mixtures of these twomonomers are suitable as monomers mentioned under a). Pure acrylamide ispreferably used.

Vinyl compounds which have a double bond capable of free radicalpolymerization and which either carry a permanently cationic charge,e.g. in the form of an ammonium group, or contain a basic group which ispresent in at least partly protonated form even under weakly acidic (pH2.5-6) conditions are suitable as monomers mentioned under b). Forexample, N,N-dialkylaminoalkyl acrylates or methacrylates orN,N-dialkylaminoalkylacrylamides or -methacrylamides and the respectivequaternization products thereof can be used. Specific examples of theseare N,N-dimethylaminoethyl acrylate, N,N-dimethylaminoethylmethacrylate, N,N-diethylaminoethyl acrylate, N,N-diethylaminoethylmethacrylate, N,N-dimethylaminopropylacrylamide,N,N-dimethylaminopropylmethacrylamide and the corresponding quaternaryammonium salts which can be prepared from the abovementioned compounds,for example by reaction with customary quaternizing reagents, such asmethyl chloride, dimethyl sulphate, epichlorohydrin or benzyl chloride,such as, for example, 2-(acryloyloxy)ethyltrimethylammonium chloride,2-(methacryloyloxy)ethyltrimethyl-ammonium chloride,3-(acrylamido)propyltrimethylammonium chloride,3-(methacrylamidopropyl)trimethylammonium chloride,2-(acryloyloxy)ethylbenzyl-dimethyl-ammonium chloride and2-(methacryloyloxy)ethylbenzyldimethylammonium chloride. Furtherexamples of suitable monomers mentioned under b) are vinylpyridine anddiallyldimethylammonium chloride. Permanently cationic monomers arepreferably used. 2-(Acryloyloxy)ethyltrimethylammonium chloride,2-(methacryloyloxy)ethyltrimethyl-ammonium chloride anddiallyldimethylammonium chloride are particularly preferred here.

For example, those having a water solubility of at least 3 g/l at 23° C.are suitable monomers mentioned under c), for example divinyl compounds,such as N,N′-methylenebisacrylamide, ethylene glycol diacrylate andethylene glycol dimethacrylate. N,N′-Methylenebisacrylamide ispreferably used.

Nonionic or anionic vinyl compounds, such as, for example, styrene,acrylic esters, methacrylic esters, acrylonitrile, methacrylonitrile,vinyl acetate, vinyl propionate, N-vinylformamide, hydroxyethyl acrylateor methacrylate, and furthermore esters of acrylic acid or methacrylicacid which can be obtained by reaction with at least 2 equivalents ofethylene oxide, and acrylic acid, methacrylic acid, itaconic acid,maleic anhydride, vinyl sulphonic acid and styrenesulphonic acid, aresuitable as monomers mentioned under d). However, the amount of themonomers mentioned under d) is preferably <3% by weight, based on thesum of a) to e).

If an anionic monomer mentioned under d) is used, the amount in relationto the amount of the cationic monomer under b) is chosen so that notmore than 0.66 mol of anionic monomer, based on 1 mol of cationicmonomer, is used. Exclusively products having an excess cationic chargeare thus obtained.

Preferably, starches based on potato starch, rice starch, wheat starch,maize starch or tapioca starch are suitable as starches mentioned undere). Usually, starches whose molecular weights have already been reducedby partial degradation and/or which have been obtained by derivatizationare preferably used instead of natural starches. Furthermore, starchesfor which both modification steps have been combined, i.e. which havebeen partially degraded and additionally derivatized, are suitable.Typical methods for starch degradation are, for example, enzymatic,oxidative, thermal or hydrolytic treatment. Examples of suitable starchderivatives are hydroxyethyl starch or cationic starch. Cationic starchis understood as meaning in general starch ethers which containquaternary ammonium groups. In the context of the present invention,hydroxyethyl starch and cationic starch are preferred, and predegradedcationic starches, each having a degree of substitution DS of >0.01, areparticularly preferred.

The polymerization is usually carried out in an aqueous medium in theneutral to acidic pH range, preferably in the pH range 2.5-6. The pH canbe adjusted to a suitable value before the beginning of thepolymerization by adding small amounts of mineral acids or organicacids, such as, for example, hydrochloric acid, sulphuric acid,phosphoric acid, formic acid or acetic acid. As a rule, the starch isinitially introduced completely in aqueous suspension or solution andthe other monomers are either added in the batch process or in the feedprocess and reacted in a free radical polymerization by means ofsuitable initiators. EP-A 194 987 describes, as a typical process,initially introducing the glutenized starch, and nonionic and cationicmonomer in the form of a mixture and initiating the polymerization byadding an initiator and carrying out said polymerization.

The invention therefore also relates to a process for the preparation ofthe starch graft polymer according to the invention, which ischaracterized in that the components a) to e) are subjected to freeradical polymerization in water in the presence of free radicalinitiators.

The invention furthermore relates to a process for the preparation of acationic starch graft polymer, characterized in that the monomers a) tod)

-   a) 20-80% by weight of acrylamide, methacrylamide or mixtures    thereof-   b) 3-20% by weight of at least one basic or cationic vinyl monomer-   c) 0-1.5% by weight of at least one bifunctional or    higher-functional crosslinking agent-   d) 0-10% by weight of at least one nonionic or anionic vinyl monomer    which differs from the monomers mentioned under a) to c)    are subjected to free radical polymerization in the presence of-   e) 15-70% by weight of at least one starch or one starch derivative,    characterized in that, in a first step, the component e), as the    grafting base, is subjected to free radical polymerization in water    with at least 30% by weight of the total amount of the component b)    and then, in a second step, the other components a), c), d) and the    remaining amount of b) are subjected to free radical polymerization    in the presence of the reaction mixture formed in the first step,    and the sum of a) to e) is 100% by weight.

The invention also relates to cationic starch graft polymers obtainableby this process.

This two-stage process is preferably carried out in such a way that, ina first step, the vinyl monomer mentioned under b) is subjected to freeradical polymerization by addition of an initiator in the presence ofthe component mentioned under e), the reaction temperature being atleast 70° C., the duration of polymerization being at least 15 min andthe amount of initiator being at least 2.0% by weight, based on thetotal amount of the monomer mentioned under b); and, in a second step,the remaining monomers are reacted in the presence of the reactionmixture formed.

The starch initially introduced in water is preferably heated to a valueabove its glutenization temperature before the first polymerization stepis started. The customary temperature range for both polymerizationsteps is 70-100° C. The temperature may also be higher if apressure-resistant reactor under superatmospheric pressure is employed.The customary duration of polymerization for the first step is 0.25-1.5h and that for the second step is 0.5-4 h. The monomers and initiatorscan in each case be added in one or in several portions over theduration of polymerization, or can also be metered in continuously.Polymerization is carried out in an inert gas atmosphere, e.g. undernitrogen.

Initiators used for the polymerization are in general free radicalinitiators, preferably peroxo or azo compounds. Examples of these arehydrogen peroxide, sodium, potassium and ammonium peroxodisulphate,di-tert-butyl peroxide, dibenzoyl peroxide, azobisisobutyronitrile,2,2′-azobis(2-methylbutyronitrile),2,2′-azobis(2,4-dimethylvaleronitrile) and2,2′-azobis(2-amidinopropane)dihydrochloride. Preferred among these areinitiators having a water solubility of >1% by weight at 23° C., suchas, for example, hydrogen peroxide and potassium and ammoniumperoxodisulphate.

Furthermore, so-called redox initiator systems, in which said freeradical initiators are used together with a reducing agent, are suitableas initiators. Examples of suitable reducing agents are sodium sulphite,sodium pyrosulphite, sodium hydrogen sulphite, sodium dithionite, sodiumformaldehyde sulphoxylate and ascorbic acid. In addition, said freeradical initiators can also be combined with heavy metal salts, such ascerium(IV), manganese or iron salts to give a suitable redox system.Ternary initiator systems consisting of free radical initiator, reducingagent and heavy metal salt are furthermore suitable.

If a binary redox system comprising free radical initiator and reducingagent is used, the reducing agent is preferably initially introducedbefore the beginning of the polymerization. An amount of 2.0-4.5% byweight, based on the total amount of monomer b), of free radicalinitiator is preferably used for the first polymerization step.

In general, the reaction conditions are preferably chosen so that atleast 50% by weight of the monomer b) are reacted during the firstpolymerization step of the two-stage process. The conversion at the endof the first step can be determined by the methods known to a personskilled in the art, such as, for example, HPLC or ¹H-NMR spectroscopy.

During the entire preparation process, the reaction mixture is usuallythoroughly mixed by means of suitable stirring or mixing units so thatthe added components are homogeneously distributed as rapidly aspossible. After the end of the addition of the monomers and theinitiator of the second polymerization step, the reaction mixture isusually allowed to continue reacting for some time in order to completethe polymerization. After this subsequent reaction time, a certainamount of initiator is preferably added again in order to polymerize assubstantially as possible the residual monomers still present in thereaction mixture. After a further subsequent reaction time, the pH ofthe resulting polymer solution can be adjusted by adding suitable bases.A value in the range 4-7 is preferably established thereby, andpreferred bases are alkali metal hydroxides and alkali metal acetates.Furthermore, buffer substances may also be added in order to stabilizethe pH over the storage time.

The concentration of the graft polymer solutions prepared by thetwo-stage process is preferably 5-25% by weight and in particular 12-22%by weight. The viscosity of the resulting solutions is preferably <5 000mPa·s measured at 23° C. by means of a rotational viscometer.

In order to increase the shelf-life of the resulting polymer solutions,a biocide may be added at the end of the preparation process in order toachieve effective protection from fungal and bacterial attack. Biocidesbased on isothiazolinones or benzoisothiazolinones, orformaldehyde-donating biocides, are preferably added for this purpose.

The cationic starch graft polymers according to the invention aresuitable in principle for ensuring the dry strength of all customarypapers, it being possible both to use them in the pulp, i.e. to add themto the paper stock prior to sheet formation, and to apply them to thesurface of the paper web, for example by means of a size press or filmpress or by spraying.

The majority of the customary paper stock systems consist of fibres,mineral fillers and water. There are also filler-free stock systems.Suitable fibres in the context of the invention are all customary types,such as bleached and unbleached, wood-free and wood-containing,wastepaper-containing and deinked stocks. Examples of customary fillersare kaolin, natural or precipitated CaCO₃, talc and titanium dioxide.

In a preferred embodiment, the polymers according to the invention areused in the pulp and owing to their cationic charge, are substantiallyabsorbed onto the cellulose fibres. The polymers according to theinvention can be used both in the acidic and in the neutral procedure.The customary added amounts (calculated as polymeric solid) are 0.05-2%by weight, based on dry paper stock. Optionally, the polymers accordingto the invention can also be combined with temporary or permanent wetstrength agents, with the result that development of increased wet anddry strength is achieved. The polymers according to the invention arevery suitable for combination with cationic wet strength agents.Examples of these are urea/or melamine/formaldehyde resins, polyamine/orpolyamidoamine/epichlorohydrin resins, glyoxalated cationicpolyacrylarrudes, as described, for example, in U.S. Pat. No. 4,605,702,and hydrophilized polyisocyanates, as described, for example, in EP-A582 166/EP-A 944 886.

In a further preferred embodiment, the polymers according to theinvention are applied to the surface of the paper web, usually by meansof suitable application units, such as a size press or film press. Thecustomary added amounts (calculated as polymeric solid) are 0.05-2% byweight, based on dry paper stock. In this application, the polymersaccording to the invention can be combined with other customary paperchemicals which are used in the surface, in particular with starches andsurface sizes. Less suitable is the combination with anionic opticalbrighteners, since the brightener effect is reduced by the cationicpolymer. When brighteners are concomitantly used, nonionic and/orcationic brighteners are preferably employed. Furthermore, the polymersaccording to the invention can also be combined with the temporary orpermanent wet strength agents described above, also in the case ofsurface application, if it is also desired to increase the wet strengthin addition to increasing the dry strength.

In addition to increasing the dry strength, i.e. the dry tensilestrength, bursting strength, cleavage strength, stiffness and bucklingresistance, the polymers according to the invention are also verysuitable for reducing dusting and picking, which is advantageous for theprintability in a number of customary printing processes. Furthermore,the dry strength agents according to the invention are suitable for usein pulp for improving the retention of fillers and of fines and foraccelerating drainage. In addition, the dry strength agents according tothe invention can advantageously be combined with the synthetic enginesizes alkylketene dimer (AKD) and alkenylsuccinic anhydride (ASA), sincethey promote the retention of these products when used in the pulp andcan also act as so-called cationic promoters; i.e. can accelerate thereaction of AKD or ASA with the cellulose fibre.

Compared with the products frequently used for increasing the drystrength and based on glyoxalated polyacrylamides, the dry strengthagents according to the invention have the advantage of a substantiallyincreased shelf-life.

EXAMPLES Preparation Examples

(all stated percentages are % by weight, unless stated otherwise)

For all preparation examples, cationic potato starches whose molecularweight had been reduced were used. Specifically, the starches used hadthe following features with regard to the method of molecular weightreduction and degree of substitution with cationic groups:

Method of reduction Degree of substitution DS Starch A Oxidative 0.018Starch B Oxidative 0.027 Starch C Oxidative 0.018

Examples 1-6 illustrate the preparation of cationic starch graftcopolymers crosslinked according to the invention. Examples 7-14illustrate the two-stage process according to the invention for thepreparation of cationic starch graft copolymers which are likewiseaccording to the invention.

Example 1

1 007 g of demineralized water are initially introduced at roomtemperature into a 2 1 plane-ground flask having jacket heating and astirrer, and 50.5 g of starch A (solids content 82.2%) are suspendedtherein with stirring. The apparatus is then placed under nitrogen.Nitrogen atmosphere and stirring are maintained for the entire furtherreaction sequence.

The starch suspension is heated to 85° C., the starch glutenizing andforming a slightly turbid solution. After 85° C. have been reached,stirring is continued for 15 min and then 0.5 g of glacial acetic acidis added. Thereafter, the following two solutions are metered inuniformly and simultaneously over 90 min at 85° C.:

Feed 1): Aqueous solution of the monomers consisting of:

-   -   78.2 g of acrylamide    -   23.35 g of 2-(acryloyloxyethyl)trimethylammonium chloride as an        80% strength aqueous solution (AETAC)    -   3.0 g of a 1% strength aqueous solution of        N,N′-methylenebisacrylamide (MBA)    -   100.0 g of demineralized water        Feed 2): Aqueous solution of the initiator, consisting of:    -   96.9 g of a 2% strength aqueous solution of ammonium        peroxodisulphate

After the end of the metering operations, stirring is continued for 45min at 85° C., after which 15.0 g of a 2% strength aqueous solution ofammonium peroxodisulphate are added for subsequent activation. Stirringis effected for a further 45 min at 85° C., after which the polymersolution is cooled to room temperature.

The pH is adjusted to 7.1 with about 7 g of 10% strength sodiumhydroxide solution, and furthermore 1.4 g of a biocide (PREVENTOL® D 2,Bayer AG) are added. Finally, the polymer solution is filtered through a100 μm polyamide filter cloth.

A moderately viscous, clear, homogeneous polymer solution is obtained.

Example 2

The procedure is as in example 1, but other amounts of initiallyintroduced water and N,N′-methylenebisacrylamide are used:

Initially introduced demineralized water 995.0 g 1% strength aqueoussolution of MBA  15.0 g

Example 3

The procedure is as in example 1, but 50.7 g of the starch B (81.9%solids content) are used.

Example 4

The procedure is as in example 2, but 50.7 g of the starch B (81.9%solids content) are used.

Example 5

The procedure is as in example 1, but 48.5 g of the starch C (85.6%solids content) are used. In contrast to example 1, 1 009 g ofdemineralized water are initially introduced.

Example 6

The procedure is as in example 2, but 48.5 g of the starch C (85.6%solids content) are used. In contrast to example 2, 997 g ofdemineralized water are initially introduced.

Example 7

1 031 g of demineralized water are initially introduced into a 2 1plane-ground flask having jacket heating and a stirrer at roomtemperature, and 49.9 g of the starch A (solids content 83.1%) aresuspended therein with stirring. The apparatus is then placed undernitrogen. Nitrogen atmosphere and stirring are maintained for the entirefurther reaction sequence.

The starch suspension is heated to 85° C., the starch glutenizing andforming a slightly turbid solution. After 85° C. have been reached,stirring is continued for 15 min, after which 0.5 g of glacial aceticacid and 23.4 g of an 80% strength aqueous solution of AETAC are added.For initiation of the 1st polymerization stage, the following solutionis metered in uniformly over 30 min at 85° C.:

Feed 1): Aqueous initiator solution consisting of:

-   -   24.2 g of a 2% strength solution of ammonium peroxodisulphate        After completion of feed 1, the 2nd polymerization stage is        carried out. For this purpose, the following solutions are        metered in uniformly over 90 min at 85° C.:        Feed 2): Aqueous acrylamide solution consisting of:    -   78.2 g of acrylamide    -   80.0 g of demineralized water        Feed 3): Aqueous initiator solution consisting of:    -   72.7 g of a 2% strength solution-of ammonium peroxodisulphate

After the end of the metering operations, stirring is continued for 45min at 85° C., after which 15.0 g of a 2% strength aqueous solution ofammonium peroxodisulphate are added for subsequent activation. Stirringis effected for a further 45 min at 85° C., after which the polymersolution is cooled to room temperature.

The pH is adjusted to 7.1 with about 7 g of 10% strength, sodiumhydroxide solution and furthermore 1.4 g of the biocide PREVENTOL® D 2(Bayer AG) are added. Finally, the polymer solution is filtered througha 100 μm polyamide filter cloth.

A slightly viscous, clear, homogeneous polymer solution is obtained.

Example 8

The procedure is analogous to example 7, but a metering time of 60 minis chosen for feed 1 and a metering time of 60 min for the simultaneousfeeds 2 and 3.

Example 9

The procedure is analogous to example 7, but the solution of AETAC ismetered in uniformly and simultaneously with feed 1 over 30 min.

Example 10

The procedure is analogous to example 7, but a different composition ofthe polymer is chosen. Specifically, in contrast to example 7, thefollowing amounts are used:

Starch A (solids content 83.1′%) 66.6 g Demineralized water (initiallyintroduced) 1 049 g AETAC as 80% strength aqueous solution 13.0 g Feed 120.8 g Feed 2: Acrylamide 72.65 g Demineralized water 75.0 g Feed 3 62.3g

Example 11

The procedure is analogous to example 7, but the starch B is used. Incontrast to example 7, the following amounts are used:

Starch B (solids content 81.9%)  50.7 g Demineralized water (initiallyintroduced)  1030 gFor a sample taken after the end of the 1st polymerization stage, amonomer conversion of 62% was determined by means of ¹H-NMR.

Example 12

The procedure is analogous to example 7, but the starch C is used. Incontrast to example 7, the following amounts are used:

Starch C (solids content 85.6%) 48.5 g Demineralized water (initiallyintroduced) 1032 g

Example 13

826 g of demineralized water are initially introduced at roomtemperature into a 2 1 plane-ground flask having jacket heating and astirrer, and 101.3 g of starch B (solids content 81.9%) are suspendedtherein with stirring. The apparatus is then placed under nitrogen.Nitrogen atmosphere and stirring are maintained for the entire furtherreaction sequence.

The starch suspension is heated to 85° C., the starch glutenizing andforming a slightly turbid solution. After 85° C. have been reached,stirring is continued for 15 min and then 0.6 g of glacial acetic acid,6.0 g of a 5% strength aqueous solution of Rongalit® C (sodiumformaldehyde sulphoxylate dihydrate) and 46.7 g of an 80% strengthaqueous solution of AETAC are added. For initiation of the 1stpolymerization stage, the following solution is metered in uniformlyover 30 min at 85° C.:

Feed 1): Aqueous initiator solution consisting of:

-   -   48.5 g of a 2% strength solution of ammonium peroxodisulphate        After completion of feed 1; the 2nd polymerization stage is        carried out. For this purpose, the following solutions are        metered in uniformly over 90 min at 85° C.:        Feed 2): Aqueous acrylamide solution consisting of:    -   156.4 g of acrylamide    -   165.0 g of demineralized water        Feed 3): Aqueous initiator solution consisting of:    -   145.3 g of a 2% strength solution of ammonium peroxodisulphate

After the end of the metering operations, stirring is continued for 45min at 85° C., after which 30.0 g of a 2% strength aqueous solution ofammonium peroxodisulphate are added for subsequent activation. Stirringis effected for a further 45 min at 85° C., after which the polymersolution is cooled to room temperature.

The pH is adjusted to 7.0 with about 11 g of 10% strength sodium,hydroxide solution, and furthermore 1.6 g of the biocide PREVENTOL® D 2(Bayer AG) are added. Finally, the polymer solution is filtered througha 100 μm polyamide filter cloth.

A moderately viscous, clear, homogeneous polymer solution is obtained.

Example 14

The procedure is analogous to example 7, but a different composition ofthe polymer is chosen. Specifically, in contrast to example 7, thefollowing amounts are used:

Demineralized water (initially introduced) 1 016 g Feed 2: Acrylamide78.2 g 1% strength aqueous solution of MBA 15.0 g Demineralized water80.0 g

Example 15 Comparative Example Analogous to EP-A 194 987

1 014 g of demineralized water are initially introduced at roomtemperature into a 2 1 plane-ground flask having jacket heating and astirrer, and 50.5 g of starch A (solids content 82.2%) are suspendedtherein with stirring. The apparatus is then placed under nitrogen.Nitrogen atmosphere and stirring are maintained for the entire furtherreaction sequence.

The starch suspension is heated to 85° C., the starch glutenizing andforming a slightly turbid solution. After 85° C. have been reached,stirring is continued for 15 min and then 0.5 g of glacial acetic acidis added. Thereafter, the following two solutions are metered inuniformly and simultaneously over 90 min at 85° C.:

Feed 1): Aqueous solution of the monomers consisting of:

-   -   78.2 g of acrylamide    -   23.35 g of AETAC as an 80% strength aqueous solution (AETAC)    -   100.0 g of demineralized water        Feed 2): Aqueous solution of the initiator, consisting of:    -   96.9 g of a 2% strength aqueous solution of ammonium        peroxodisulphate

After the end of the metering operations, stirring is continued for 45min at 85° C., after which 15.0 g of a 2% strength aqueous solution ofammonium peroxodisulphate are added for subsequent activation. Stirringis effected for a further 45 min at 85° C., after which the polymersolution is cooled to room temperature.

The pH is adjusted to 7.1 with about 7 g of 10% strength sodiumhydroxide solution, and furthermore 1.4 g of the biocide PREVENTOL® D 2(Bayer AG) are added. Finally, the polymer solution is filtered througha 100 μm polyamide filter cloth.

A moderately viscous, clear, homogeneous polymer solution is obtained.

TABLE 1 Physical parameters of the polymer solutions of examples 1-15 pHSolids content Viscosity* at 23° C. Example established (%) (mPa · s) 17.1 10.3 390 2 7.3 10.2 90 3 7.1 10.3 120 4 7.1 10.3 100 5 7.0 10.0 60 67.0 10.2 3000 7 7.1 10.0 20 8 7.0 10.0 25 9 7.0 9.9 20 10 7.1 10.1 90 117.0 9.9 10 12 7.0 9.9 20 13 7.0 18.1 260 14 7.0 10.0 20 15 7.1 10.3 150*measured using a Haake VT 5L rotational viscometer (L3 spindle)

Use Examples

(The stated amounts in the following examples are specified aspercentages by weight, unless stated otherwise. All stated amountsrepresent the content of dry polymer, based on dry paper.)

0.5% or 1.5% of each of the polymers from the preceding preparationexamples were added with stirring to a wastepaper-containing stocksuspension having a consistency of about 5 g/l, prepared from rebeaten,CaCO₃-containing lining cardboard. After a mixing time of 2.5 minutes,0.2% of a retention. aid (RETAMINOL E, Bayer AG) was added. After afurther mixing time of 10 s, sheets were formed using a Rapid-Koethenlaboratory sheet former. These were dried for 7 min in the connecteddryer and then conditioned overnight at 23° C. and 50% relativehumidity. The ash content of the laboratory sheets without polymeraddition, determined by ashing at about 900° C., was 11-13%. The basisweights of the sheets formed were in the range 65-85 g/m².

The sheets thus formed measured with regard to dry tensile strengthusing an apparatus from Frank and with regard to the bursting strengthusing an apparatus from Lorentzen & Wettre. From these data, therelative increase in the dry tensile strength and in the burstingstrength was calculated, relative to a comparative sample withoutaddition of a corresponding dry strength agent. The following formula,which is shown here by way of example for the dry tensile strength, wasused for the calculation:

${{Relative}\mspace{14mu}{increase}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{dry}\mspace{14mu}{tensile}\mspace{14mu}{strength}} = {{\Delta\;{DTS}} = {\frac{\left( {{SDTA}_{{{Test}\mspace{11mu}{sample}}\;} - {SDTS}_{{Zero}\mspace{14mu}{sample}}} \right)}{{SDTS}_{{Zero}\mspace{14mu}{sample}}} \cdot {100\lbrack\%\rbrack}}}$where:

-   SDTS=Standardized dry tensile strength in N-   Zero sample=Laboratory sheet without dry strength agent-   Test sample=Laboratory sheet with dry strength agent to be tested

All measured values of the dry tensile strength and of the burstingstrength were based on a basis weight of 80 g/m², with the result thatfive standardized dry tensile strength was calculated from the measureddry tensile strength:

${{Standardized}\mspace{14mu}{dry}\mspace{14mu}{tensile}\mspace{14mu}{strength}} = {{SDTS} = \frac{\left( {{DTS} \cdot {BW}} \right)}{80}}$where:

-   DTS=Measured value of the dry tensile strength-   BW=Basis weight of the associated laboratory sheet in g/m²

Analogous procedures were used for calculating the standardized burstingstrength or the relative increase in the bursting strength.

The values for the relative increase in the dry tensile strength (ΔDTS)or the relative increase in the bursting strength (ΔBS) are listed intable 2.

TABLE 2 Strength parameters of papers which were treated with polymersolutions from examples 1-15 Added amount Δ DTS Δ BS Example (%) (%) (%)1 0.5 6.3 24.5 1.5 20.7 40.1 2 0.5 9.8 25.0 1.5 22.5 36.6 3 0.5 7.7 16.51.5 21.6 30.1 4 0.5 3.9 21.2 1.5 19.7 35.8 5 0.5 6.3 19.9 1.5 22.8 32.26 0.5 17.4 18.5 1.5 20.7 35.3 7 0.5 11.4 22.9 1.5 30.9 40.5 8 0.5 17.926.6 1.5 34.6 37.9 9 0.5 16.3 17.6 1.5 30.2 35.9 10 0.5 10.7 23.4 1.518.5 32.4 11 0.5 25.0 27.6 1.5 23.2 39.6 12 0.5 17.0 18.7 1.5 34.4 44.513 0.5 13.8 19.4 1.5 15.0 34.6 14 0.5 16.1 25.4 1.5 21.2 39.3 15 0.5-0.2 11.3 1.5 13.1 29.8

Both the use of a crosslinking agent (examples 1-6) and the use of thetwo-stage process according to the invention (examples 7-14) lead, whenthe same amounts are used, to higher strength parameters compared withthe polymer solutions (example 15) known from the prior art.

The following preparation examples 16-20 show further possibilities forthe preparation of the polymers according to the invention and forcarrying out the processes according to the invention. In all cases,polymer solutions which likewise increased the dry strength of paper ina very good manner in pulp or surface applications were obtained.

Example 16

The procedure is as in example 1, but in contrast the starch C (solidscontent 85.6%) and acrylic acid are used as further monomer in feed 1.The amounts used are as follows:

Starch C 48.5 g Demineralized water (initially introduced)  995 g Feed1): Aqueous solution of the monomers consisting of:  74.7 g ofacrylamide 23.35 g of AETAC as 80% strength aqueous solution  3.5 g ofacrylic acid  3.0 g of a 1% strength aqueous solution of MBA 100.0 g ofdemineralized water 10% strength sodium hydroxide solution 22.0 g

A clear, homogeneous polymer solution, having a pH of 5.3, a solidscontent of 10.3% and a viscosity of 2 390 mPa·s is obtained.

Example 17

1 005 g of demineralized water are initially introduced at roomtemperature into a 2 1 plane-ground flask having jacket heating and astirrer, and 49.9 g of starch A (solids content 83.1%) are suspendedtherein with stirring. The apparatus is then placed under nitrogen.Nitrogen atmosphere and stirring are maintained for the entire furtherreaction sequence.

The starch suspension is heated to 85° C., the starch glutenizing andforming a slightly turbid solution. After 85° C. have been reached,stirring is continued for 15 min and then 2.0 g of glacial acetic acidand 0.65 g of sodium formaldehyde sulphoxylate dihydrate are added.Thereafter, the following two solutions are metered in uniformly andsimultaneously over 90 min at 85° C.:

Feed 1): Aqueous solution of the monomers consisting of:

-   -   78.2 g of acrylamide    -   23:35 g of diallyldimethylammonium chloride as an 80% strength        aqueous solution    -   3.0 g of a 1% strength aqueous solution of MBA    -   100.0 g of demineralized water        Feed 2): Aqueous solution of the initiator, consisting of:    -   96.9 g of a 2% strength aqueous solution of ammonium        peroxodisulphate

After the end of the metering operations, stirring is continued for 45min at 85° C., after which 15.0 g of a 2% strength aqueous solution ofammonium peroxodisulphate are added for subsequent activation. Stirringis effected for a further 45 min at 85° C., after which the polymersolution is cooled to room temperature.

The pH is adjusted to 7.1 with about 14 g of 10% strength sodiumhydroxide solution, and furthermore 1.4 g of the biocide PREVENTOL® D 2(Bayer AG) are added. Finally, the polymer solution is filtered througha 100 μm polyamide filter cloth.

A clear, homogeneous polymer solution having a solids content of 10.0%and a viscosity of 10 mPa·s is obtained.

Example 18

The procedure is analogous to example 1, but, instead of MBA, ethyleneglycol dimethacrylate is used as a crosslinking agent in feed 1. Incontrast to example 1, the following amounts are used:

Starch A (solids content 83.1 %) 49.9 g Demineralized water (initiallyintroduced) 995 g 1% strength aqueous emulsion of ethylene glycoldimethacrylate 15.0 g (emulsified by addition of 0.1% of Nadodecylsulphate)

A clear, homogeneous polymer solution having a pH of 7.1, a solidscontent of 10.2% and a viscosity of 120 mPa·s is obtained.

Example 19

The procedure is analogous to example 7, but a different composition ofthe polymer is chosen. Specifically, in contrast to example 7, thefollowing amounts are used:

Feed 2: Acrylamide 73.4 g Acrylic acid 4.8 g Demineralized water 80.0 g10% strength NaOH about 30 g

A clear, homogeneous polymer solution having a pH of 7.0, a solidscontent of 9.8% and a viscosity of 25 mPa·s is obtained.

Example 20

1 022 g of demineralized water are initially introduced at roomtemperature into a 2 1 plane-ground flask having jacket heating and astirrer, and 49.9 g of starch A (solids content 83.1%) are suspendedtherein with stirring. The apparatus is then placed under nitrogen.Nitrogen atmosphere and stirring are maintained for the entire furtherreaction sequence.

The starch suspension is heated to 85° C., the starch glutenizing andforming a slightly turbid solution. After 85° C. have been reached,stirring is continued for 15 min and then 0.5 g of glacial acetic acid,0.15 g of Rongalit® C (sodium formaldehyde sulphoxylate dihydrate) and25.6 g of a 65% strength aqueous solution of diallyldimethylammoniumchloride are added. For initiation of the 1^(st) polymerization stage,the following solution is metered in uniformly over 30 min at 85° C.:

Feed 1): Aqueous initiator solution consisting of:

-   -   24.2 g of a 2% strength solution of ammonium peroxodisulphate        After completion of feed 1, the 2nd polymerization stage is        carried out. For this purpose, the following solutions are        metered in uniformly over 90 min at 85° C.:        Feed 2): Aqueous acrylamide solution consisting of:    -   80.3 g of acrylamide    -   85.0 g of demineralized water        Feed 3): Aqueous initiator solution consisting of:    -   72.7 g of a 2% strength solution of ammonium peroxodisuiphate

After the end of the metering operations, stirring is continued for 45min at 85° C., after which 15.0 g of a 2% strength aqueous solution ofammonium peroxodisulphate are added for subsequent activation. Stirringis effected for a further 45 min at 85° C., after which the polymersolution is cooled to room temperature.

The pH is adjusted to 7.0 with about 6 g of 10% strength sodiumhydroxide solution, and furthermore 1.4 g of the biocide PREVENTOL® D 2(Bayer AG) are added. Finally, the polymer solution is filtered througha 100/lm polyamide filter cloth.

A clear homogeneous polymer solution having a pH of 7.0, a solidscontent of 10.0% and a viscosity of 10 mPa·s is obtained.

1. A process for manufacture of a dry strength paper, comprising:applying to a paper or a paper pulp at least one cationic starch graftpolymer and a grafting base; wherein the at least one cationic starchgraft polymer is composed of monomers a) to d): a) 20-80% by weight ofan acrylamide, a methacrylamide or mixtures thereof, b) 3-20% by weightof at least one basic or cationic vinyl monomer, c) 0.005-1.5% by weightof at least one bifunctional or higher functional crosslinking agent, d)0-10% by weight of at least one nonionic or anionic vinyl monomer whichdiffers from the monomers mentioned under a) to c); wherein the graftingbase is composed of grafting base e): e) 15-70% by weight of at leastone starch or starch derivative, and wherein the sum of a) to e) is 100%by weight; and wherein the cationic starch graft polymer is prepared bya process comprising: free radical polymerization in water of thegrafting base e) and at least 30% by weight of the total amount ofmonomer b) to form a reaction mixture; and free radical polymerizationof monomers a), c), d) and the remaining amount of b) in the presence ofthe reaction mixture to form the cationic starch graft polymer.
 2. Theprocess according to claim 1, wherein monomer a) is the acrylamide. 3.The process according to claim 1, wherein the at least one basic orcationic vinyl monomer of component b) has a double bond capable of freeradical polymerization and contains either a permanently cationic chargeor a basic group that is present in completely or partly protonated formunder weakly acidic conditions.
 4. The process according to claim 1,wherein the at least one bifunctional or higher-functional crosslinkingagent of component c) is a water-soluble divinyl compound.
 5. Theprocess according to claim 1, wherein the grafting base of component e)is a starch whose molecular weight has been reduced by partialdegradation and/or is derivatized.
 6. The process according to claim 1,wherein the monomers a) to d) and the grafting base e) are subjected tofree radical polymerization in water in the presence of free radicalinitiators.
 7. A method for imparting dry strength to paper, comprisingadding a cationic starch graft polymer to a paper stock suspension priorto sheet formation or applying the cationic starch graft polymer to asurface of a paper web, wherein the cationic starch graft polymer isformed by polymerizing monomers a) to d): a) 20-80% by weight ofacrylamide, methacrylamide or mixtures thereof, b) 3-20% by weight of atleast one basic or cationic vinyl monomer, c) 0.005-1.5% by weight of atleast one bifunctional or higher-functional crosslinking agent, d) 0-10%by weight of at least one nonionic or anionic vinyl monomer whichdiffers from the monomers mentioned under a) to c), and a grafting basee): e) 15-70% by weight of at least one starch or one starch derivative,wherein the sum of a) to e) is 100% by weight; and wherein the cationicstarch graft polymer is prepared by a process comprising: free radicalpolymerization in water of the grafting base e) and at least 30% byweight of the total amount of monomer b) to form a reaction mixture; andfree radical polymerization of monomers a), c), d) and the remainingamount of b) in the presence of the reaction mixture to form thecationic starch graft polymer.